So as a last throw of the dice, while the array was stopped, I changed the Disk 1 settings file type to Auto. This magically got rid of the orange unlock icon, hurrah! Started a rebuild and went to bed; surely this would produce the result?
Nope.
The rebuild completed, but now we had a different error. Unmountable: No File System.
Stopped the array, removed the disk and looked at the underlying partition on the disk. It was formatted to xfs, but 0 bytes. So somehow the rebuild has just written zeroes to it.
It looks like this is beyond any hope of recovery; I believe I have archive copies of 95% of my data, but it is on unprotected (ha!) disks. I need to blow away this whole config and start from scratch, as I just don't trust it now. We may never know how it got to this stage, but it IS possible to lose a disk in an Unraid array and not be able to get it back. Think I'll choose a second parity disk this time...